A Closer Look at Vehicle Data Communications A … Closer Look at Vehicle Data Communications T ... Comfort) • SBI (Simple Bus Interface) ... specific ID information codes when

22 GEARS April 2010

A Closer Look at Vehicle Data Communications

Through the years we’ve come to depend on the infor-mation we can col-

lect with a scan tool. And through those years, the data communication systems or data buses have become more complex. With OBD-II, many of us hoped that the data bus would be standardized and that all our problems would be solved. No such luck.

Today’s vehicles use a variety of data transmission pro-tocols. This forces scan tool manufac-turers to constantly adapt their software to those changes. This can be quite a challenge, even if they’re working with a single brand of vehicle. Let’s look at GM as an example: GM has used all of these data communication protocols over the years:

UART (Universal •Asynchronous Receive and Transmit)E&C (Entertainment and •Comfort)SBI (Simple Bus Interface)•SPI (Serial Peripheral •Interface)LIN (Local Interconnect •Network)Keyword 81,82 and 2000•Class 2•CAN (Controller Area •Network) or GMLAN (General Motors Local Area Network)

With the average GM vehicle containing more than a mile of wire, dozens of computers, hundreds of con-nectors, and thousands of terminals, the need for a standardized diagnostic system became imperative. A new sys-tem was introduced for the 2010 model year on the Chevrolet Camaro, Cadillac

SRX, GMC Terrain, Buick Allure and LaCrosse. The new platform, known as the Global Diagnostic System or GDS, will be introduced in all GM vehicles in 2010/2011 models.

The new diagnostic system still uses several communication protocols from previous models, but it will also

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24 GEARS April 2010


vary greatly when it comes to diagnos-tic processes and equipment. To under-stand the typical Global Architecture (Global A) we’ll need to look at the data bus and its operation. The stan-dardized, 16-pin DLC connector that you connect to your scan tool will have these pin assignments (figure 1):

Pin 1 — Low Speed GM LANPin 3 — Mid Speed GM LAN; +

terminalPin 4 — Scan Tool GroundPin 5 — Common GroundPin 6 — High Speed GM LAN; +

terminalPin 11 — Mid Speed GM LAN; –

terminalPin 12 — Expansion Bus High

Speed GM LANPin 13 — Expansion Bus High

Speed GM LANPin 14 — High Speed GM LAN;

– terminalPin 16 — Scan Tool Power B+

The primary function of the vehi-cle data bus is still communication between the different control modules. GM has used several different data bus

configurations, including:• Ring Network — Connects each module through a serial data line.• Star Network — Serial data lines from all the modules are connected at a single point; typically a splice pack.• Ring/Star Hybrid — A combination of the two common bus configurations.

Let’s take a look at some of the basics for a GM data bus system:

Power Moding — Power moding was introduced on GM vehicles many years ago to reduce electrical loads, wiring, and improve module response time during wakeup. Years ago, GM modules required a wakeup signal for the module to come on line. This was typically accomplished using an input from the ignition switch.

In today’s vehicles, wakeup is accomplished by an assigned Power Mode Master module (PMM). The PMM is typically the BCM or DIM. The PMM receives the input from the ignition switch and then relays that information to the other modules through serial data on the data bus. Messages to the modules include:

Off/A Sleep — Modules need •to power down. Off Awake — Modules need •to wake up and get ready for work.RAP (Retained Accessory •Power) — Modules that are used after the key is turned off but before the door is opened should stay awake for a set period of time; all others should sleep.Accessory — Modules that •are used when the key is in the accessory position should stay awake.Run — All modules should be •awake.Crank — Modules that are •used for cranking the engine should wake up while all oth-ers should continue to sleep.

Gate Way Module — Since 1986, many GM vehicles have used a master module to control data transmission on the bus. With a CAN/GM LAN network, the BCM typically acts as the gateway or master control module. The

Figure 2

Figure 4

Figure 3

Figure 5

GEARS April 2010 25

gateway module translates and priori-tizes the messages sent on the bus. In other words, the BCM acts as the inter-preter and traffic cop for the system.

CAN/GM LAN — The CAN data bus isn’t new, but it’s becoming more common throughout the industry. GM has its own version of the CAN sys-tem known as the GM LAN. The GM LAN supports three types of bus con-figurations: low speed (33.3 kilobits per second), mid speed (95.2 kilobits per second) and high speed (500 kilobits per second) data transmission.

The high speed bus (figure 2) con-sists of a twisted wire pair that transmits a voltage signal that switches high and low to create the data signal. The high speed GM LAN bus is terminated with a pair of 120 ohm resistors. One line is driven high (3.5 volts) while the other data line is pulled low (1.5 volts) in a specific sequence to create the signal.

With a CAN-type system, each message is assigned a unique identi-fier to classify the type message and its priority. Each module on the bus processes only the messages related to the identifiers stored in the module’s programmed listing. If more than one module is trying to communicate at the same time, priority is assigned to the message that has the greatest impor-tance.

The mid speed GM LAN bus (fig-ure 3) is very similar in operation to the high speed circuit, with one exception: the controllers attached to it. The mid speed GM LAN uses the same type of message transmission protocols that the high speed system does. The mid speed bus consists of a twisted wire pair that transmits a voltage signal that switches high and low to create the data signal. The mid speed GM LAN bus is termi-nated with resistors.

The low speed GM LAN is a single-wire circuit that replaced the class 2 data circuits on many GM vehicles (Figure 4). The GM low speed LAN is bidirectional and operates at three times the speed of a class 2 communication circuit. The circuit toggles from 0-5 volts. To wake up the modules on the bus, the voltage level is pulsed to 10-12 volts. The modules are connected in parallel on the low speed bus.

LIN — The Local Interconnect

Network (LIN) is used to reduce the cost of data bus communications. The LIN is a single-wire network that toggles between system voltage and ground to produce series of voltage pulses.

Multiple LIN circuits are used in GM vehicles. A LIN circuit doesn’t connect directly to the DLC; it commu-nicates with various sensors, actuators and modules on its own circuit.

An example of a LIN circuit would be the power windows on a late model GM (figure 5). The window switch commands the driver or passenger door control module to open or close the window. The module performs the

commanded operation while communi-cating with other modules regarding the action it performed.

GM Expansion Bus — This bus circuit can be referred to by many dif-ferent names, but the function is the same. The expansion bus is designed to reduce the congestion on the high speed bus. By adding a bus that is designed and functions the same as the high speed bus reduces the amount of traffic on the primary high speed bus. This is like adding parallel freeway to reduce traffic congestion in a large city.

With the global architecture, lots of things have changed. For decades tech-

26 GEARS April 2010


nicians have used the Tech 2 scan tool to help diagnose vehicles. With Global A vehicles, the Tech 2 will no longer function. GM now uses a PC-based program known as GDS or Global Diagnostic System (figure 6) to inter-face with the vehicle. This means you need to ask your scan tool manufacturer if your scan tool will be compatible with this new system. Lots of other areas have also changed, including:

Swapping computers for test •purposes can cause major problems. While this isn’t new with some GM vehicle platforms, it’s an issue with all Global A applications. Swapping an ECM, BCM, EBCM, SDM, ECC, IPC as well as many other modules can prevent the engine from starting or set trouble codes. In some instances you won’t be able to clear these codes.

The modules are all coded with specific ID information codes when they’re programmed at the assembly plant. So modules that have nothing to do with starting the engine can prevent it from starting. In addition, other symptoms may occur such as the IPC odometer reading “----,” the radio displays “Locked,” or “service the theft

system” is displayed on the DIC.Not only do the Global A vehicles

use controller security codes to prevent vehicle theft, they also use a new type of coding known an Environmental ID. This type of coding is designed to pre-vent someone from stealing a vehicle by swapping modules and keys. The Immobilizer Master (BCM) identifies itself to the other modules. If it has the correct ID, the other modules will respond with a specific Environmental ID code. The BCM will look at all the modules’ Environmental IDs and, if they’re correct, it’ll allow the vehicle to operate.

Reprogramming is sequential. •This means that several controllers are updated at the same time in a specific sequence to prevent software conflicts. Typical programming now requires a lot of data to be transferred (typically 10-40 MB). Programming will take much longer than in the past (up to an hour).

In addition, to protect the modules from unauthorized programming (non GM), the assembly plant uses a process known as Seed and Key. The assembly plant installs a specific Seed value

during plant programming. When the module is connected to reprogramming equipment, that equipment must pro-vide a specific Key value or the module won’t accept the program.

Many modules require a spe-•cific setup procedure to relearn compo-nent and system values.

Many modules require a secu-•rity code configuration procedure for the module to operate properly. If the configuration codes aren’t installed, some modules will lock out immedi-ately, while others will initially oper-ate and then lock out after several key cycles or after battery power has been removed. This could lead to a customer complaint because the engine won’t start or other conditions after you per-formed a repair.

Well as you can see times are still changing… until next time, remember: “In the middle of difficulty lies oppor-tunity.”

Figure 6

Facts The Beast The Animal

Manufactured using a 2 Pc. (middle and low) carbon steel. The wall thickness of the shell body is increased for added strength Yes No

Spline is case hardened to better withstand torque during engagement Yes NoBetter and stronger than conventional OEM design Yes NoThe sungear shell assembly includes an injection molded plastic washer. The plastic washer has the tabs inserted into holes formed in the neck section of the sungear shell to form the complete sungear shell assembly

Yes No

Strengthened and reinforced the neck to reduce stress and to eliminate high failure rate at the neck section Yes No

Invention protected by US patent Yes No

GENERAL MOTORS 700-R4/4L60, 4L60-E, 4L65-E

SUNSHELLIt has been brought to our attention that a product called “The Animal” has been recently intro-duced to the market and has created some confusion between this new product and our redesigned sungear shell “THE BEAST”.

“THE BEAST” sungear shell is made solely by Parker Seal Aftermarket Products under US patent No. 6,561,944. We believe it is our responsibility to inform you about the main differences between these two products.

Neck Area“The Animal”

Neck Area“The Beast”

Top View“The Animal”

Top View“The Beast”

Marking - Patent No.“The Beast”

Marking - Part No.“The Beast”

Sungear Shell Assembly“The Beast”

Case Hardened Spline“The Beast”

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A Closer Look at Vehicle Data Communications A … Closer Look at Vehicle Data Communications T ... Comfort) • SBI (Simple Bus Interface) ... specific ID information codes when